Chapter Nine Electrochemistry Applications. Copyright © Houghton Mifflin Company. All rights reserved.9 | 2 Batteries and Fuel Cells We’ve seen examples

Chapter Nine Electrochemistry

Applications

Copyright © Houghton Mifflin Company. All rights reserved. 9 | 2

Batteries and Fuel Cells

We’ve seen examples of batteries in our examination of electrochemical cells. Following are common examples that employ such methods.


A Flashlight Battery Consists of a Zinc Cup Anode and a Cathode Made of an Inert Carbon Electrode Immersed in a Paste

containing MnO2 and Mn2O3

Zn(s) + 2 MnO2(s) + H2O Zn2(aq) + Mn2O3(s) + 2 OH-(aq)


The Atomic-level View of the Reactions Involved in a Car Battery Shows Lead's Oxidation to PbSO4 at the Anode and Reduction of

PbO2 to PbSO4 at the Cathode

Pb(s) PbSO4(s)

PbO2(s) PbSO4(s)

Balance this reaction


A Hydrogen-Oxygen Fuel Cell in an Environmentally Friendly Method for Producing Electrical Energy.

The Only By-Product is Water


In the Fuel Cell, Electron Transfer Is Indirect

O2(g) + 2 H2O(l) + 4 e- 4 OH-(aq)

H2(g) + 2 OH-(aq) 2 H2O(l) + 2 e-

Combine these and compute the cell voltage


Electrolysis

• As the name defines, “lysis” is to break apart, thus electro-lysis is the breaking of of materials using electricity

• An electrolytic cell is a batter run in reverse. This process regenerates the battery in your car while the engine is running.

• Question: Can one produce sodium and chlorine gas from a salt water solution?


Salt Water ElectrolysisIn a solution of NaCl, we have Na+, Cl- and H2O. The possible reactions are:

Na+(aq) + e- Na(s) Eo = -2.71 V2 Cl-(aq) Cl2(g) + 2 e- Eo = -1.36 V2 H2O(l) + 2 e- H2(g) + 2 OH-(aq) Eo = -0.83 V2 H2O (l) O2(g) + 4 H+(aq) + 4 e- Eo = -1.23 V

Electrolysis will begin when the minimum threshold voltage is achieved.To produce Na and Cl2 2.71V + 1.36 V = 4.07 V are required.However, when the threshold of 0.83 V + 1.23 V = 2.06 V is achieved, water electrolysis will begin first! So, it is not possible to produce Na and Cl2 this way.


Production of MetalsThe production of metals usually involves electrolysis of the molten salt. Here, NaCl is kept above the melting point of 800oC


Schematic of the Apparatus for the Electrolytic Production of Aluminum Showing Molten Aluminum

Sinking to the Bottom of the Tank


Corrosion Prevention

• Corrosion of metals is of great concern in everything from power lines and utility poles to bridges and buildings.

• Active metals used in construction should be protected in some form.


Examining the Location of Corrosion Provides Insight into the Atomic-level Events Involved in Corrosion

When placed in a solution of phenolphthalein and [Fe(CN)6]-3, the solution turns pink in the presence of OH-1 ions and blue in the presence of oxidized iron, Fe+2.

Oxidation occurs at defect points where exposure to the metals is greatest.Fe Fe+2 + 2 e- at the blue areas and O2(g) + 2 H2O + 4 e- 2 OH- in the pink areas giving an overall reaction of the production of rust.2 Fe + O2 2 FeO


The Site of Oxidation (electron source) Must Be Connected to the Site of Reduction

(electron sink) by a Conductive Material


Sacrificial Anodes

Metal such as iron is optimal for many reasons including it’s low cost and availability, conductive properties, and metallic behavior. However, it suffers from being a fairly active metal. How does one prevent the iron from decomposing?

Answer: Place the iron in contact with another, more active metal. Being more active, the more active metal will oxidize preferentially to iron. Such metals are called “sacrificial anodes”


Underground Steel PipesHere, Mg is more active than the pipe and will corrode first. Attack on the pipe is prevented by the transfer of electrons by the magnesium metal. Copper is very inactive and will not corrode.


A Steel Utility Pole Is Connected to a Magnesium Stake. The Magnesium Feeds Electrons to the Iron in the Steel Pole via the

Conducting Connector, Preventing Oxidation of the Iron


Oxide Coating Protection

• Aluminum is an active metal, however, when exposed to air, it forms a tough Al2O3 coating that protects the interior metal.

• Iron nails are often “galvanized” by coating with a layer of zinc oxide, ZnO which protects the interior metal.

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Chapter Nine Electrochemistry Applications. Copyright © Houghton Mifflin Company. All rights reserved.9 | 2 Batteries and Fuel Cells We’ve seen examples